diff --git a/src/dice_gen.rs b/src/dice_gen.rs
index 882aff7..53117c7 100644
--- a/src/dice_gen.rs
+++ b/src/dice_gen.rs
@@ -1,40 +1,80 @@
-pub struct DiceGen {
+pub(crate) trait DiceGen {
+    /// Returns two random dice with values each between 1 and 6. Not sorted by size.
+    fn roll(&mut self) -> (usize, usize);
+}
+
+pub(crate) struct FastrandDice {
     generator: fastrand::Rng,
 }
 
-impl DiceGen {
+impl DiceGen for FastrandDice {
+    /// Returns two random dice with values each between 1 and 6. Not sorted by size.
+    fn roll(&mut self) -> (usize, usize) {
+        let random = self.generator.usize(0..36);
+        let die1 = random / 6 + 1;
+        let die2 = random % 6 + 1;
+        (die1, die2)
+    }
+}
+
+impl FastrandDice {
     #[allow(dead_code)]
-    pub fn new() -> DiceGen {
-        DiceGen {
+    pub fn new() -> FastrandDice {
+        FastrandDice {
             generator: fastrand::Rng::new(),
         }
     }
 
     #[allow(dead_code)]
-    pub fn with_seed(seed: u64) -> DiceGen {
-        DiceGen {
+    pub(crate) fn with_seed(seed: u64) -> FastrandDice {
+        FastrandDice {
             generator: fastrand::Rng::with_seed(seed),
         }
     }
+}
 
-    /// Returns two random dice with values each between 1 and 6. Not sorted by size.
-    #[allow(dead_code)]
-    pub fn roll(&mut self) -> (usize, usize) {
-        let random = self.generator.usize(0..36);
-        let die1 = random / 6 + 1;
-        let die2 = random % 6 + 1;
-        (die1, die2)
+#[cfg(test)]
+/// Use this for unit tests where you want to control the dice.
+pub(crate) struct DiceGenMock {
+    dice: Vec<(usize, usize)>,
+    no_calls: usize, // how often `roll` has been called.
+}
+
+#[cfg(test)]
+impl DiceGen for DiceGenMock {
+    fn roll(&mut self) -> (usize, usize) {
+        let dice = self.dice[self.no_calls];
+        self.no_calls += 1;
+        dice
+    }
+}
+
+#[cfg(test)]
+impl DiceGenMock {
+    pub(crate) fn new(dice: &[(usize, usize)]) -> DiceGenMock {
+        DiceGenMock {
+            dice: dice.to_vec(),
+            no_calls: 0,
+        }
+    }
+
+    pub(crate) fn assert_all_dice_were_used(&self) {
+        assert_eq!(
+            self.dice.len(),
+            self.no_calls,
+            "Not all dice of the mock have been used"
+        );
     }
 }
 
 #[cfg(test)]
-mod tests {
-    use crate::dice_gen::DiceGen;
+mod fastrand_dice_tests {
+    use crate::dice_gen::{DiceGen, FastrandDice};
 
     #[test]
     fn all_numbers_are_occurring() {
         // Given
-        let mut gen = DiceGen::with_seed(123);
+        let mut gen = FastrandDice::with_seed(123);
         let mut count1 = [0, 0, 0, 0, 0, 0];
         let mut count2 = count1;
         // When
@@ -52,3 +92,33 @@ mod tests {
         }
     }
 }
+
+#[cfg(test)]
+mod dice_gen_mock_tests {
+    use crate::dice_gen::{DiceGen, DiceGenMock};
+
+    #[test]
+    // #[should_panic]
+    fn roll_returns_given_dice() {
+        let mut dice_gen = DiceGenMock::new(&[(3, 2), (1, 6)]);
+        assert_eq!(dice_gen.roll(), (3, 2));
+        assert_eq!(dice_gen.roll(), (1, 6));
+    }
+
+    #[test]
+    // #[should_panic]
+    #[should_panic(expected = "Not all dice of the mock have been used")]
+    fn assert_all_dice_were_used() {
+        let mut dice_gen = DiceGenMock::new(&[(3, 2), (1, 6)]);
+        dice_gen.roll();
+        dice_gen.assert_all_dice_were_used();
+    }
+
+    #[test]
+    #[should_panic(expected = "index out of bounds: the len is 1 but the index is 1")]
+    fn panics_when_roll_is_called_too_often() {
+        let mut dice_gen = DiceGenMock::new(&[(3, 2)]);
+        dice_gen.roll();
+        dice_gen.roll();
+    }
+}